Role of dopamine–adenosine interactions in the brain circuitry regulating effort-related decision making: insights into pathological aspects of motivation

Dopaminergic involvement in activational aspects of motivation Similar to other psychological constructs such as emotion and cognition, motivation is not a simple or unitary phenomenon. Motivation is a complex and multifacted process that includes many diverse components. Some aspects of motivation are related to sensations of internal and external stimuli, while other aspects of motivation are related to motor function [1]. Motivated behavior takes place in phases that represent different degrees of physical or psychological distance from the primary motivational stimulus (i.e., appetitive vs consummatory; instrumental vs consummatory; see [2–4]). Moreover, motivation theory and research has emphasized for several years that there are ‘directional’ and ‘activational’ aspects of motivation [5–7]. Directional aspects refer to the observation that the behavior of animals is directed towards or away from particular motivational stimuli. In addition, it is evident that motivated behavior can be characterized by persistence, vigor and high levels of work output; these activational aspects of motivated behavior are highly adaptive because they enable organisms to overcome challenges or work-related response costs that separate them from significant stimuli such as food [4,8–13]. While foraging in the wild, animals can invest considerable time and can cover large areas of space in order to gain access to food or other primary motivational stimuli. Laboratory experiments have shown that animals can climb barriers, run in mazes or press levers on schedules with high input ratio requirements, in order to gain access to motivational stimuli such as food. Under some conditions the presentation of motivational stimuli can generate heightened, even excessive, levels of motor activity. In humans, impairments in behavioral activation can manifest themselves as energy-related symptoms such as psychomotor slowing, anergia and fatigue, which are features of depression, and can also be observed in other psychiatric or n eurological disorders [11]. In addition to studying these behavioral processes involved in activational aspects of motivation, neuroscientists have also focused upon the brain mechanisms that are potentially involved. Several brain areas have been investigated, but one of the neural systems most closely associated with behavioral activation is the dopamine (DA) innervation of the nucleus accumbens [8–11,14,15]. Although the mesolimbic DA system has consistently been linked to aspects of drug reinforcement, and is often referred to as some type of ‘pleasure’ or ‘reward’ center, recent Role of dopamine–adenosine interactions in the brain circuitry regulating effort-related decision making: insights into pathological aspects of motivation